To keep a window of a motor vehicle free from visual obstructions, various devices are known which guide a window wiper over the window whenever visual obstructions which may be cleaned off are present. Such automatically activated window wiper systems are advantageous in particular when the wetting of the window with water drops varies in intensity as the result of variable precipitation, for example.
In one variant of a system for determining the wetting, light from a light source is injected into the window in such a way that the light is cast back and forth in total reflection between the inner and outer surfaces of the window. If wetting, for example in the form of a water drop, is present in the region of these total reflections, the reflection is disturbed and a portion of the light exits from the window pane. A light sensor that registers light which originates from the light source and which is totally reflected at another location at the window is able to register such a loss of light, and a signal may thus be provided which is a function of wetting of the window.
U.S. Pat. No. 7,259,367 B2 discloses a sensor system in which water drops on the window are determined based on a comparison of camera data of a window, with and without illumination by a controlled light source.
DE 10 2004 015 040 A1 discloses a sensor system based on the principle of total reflection, using a camera, the camera supplying image data for a driver assistance system, and a portion of an image sensor surface in the camera being reserved for determining raindrops on the window.
A sensor system which is based on irradiating the inner surface of the window in such a way that a small portion of the light may be reflected at the surface of the window, while a large portion of the light may pass through the window, and the light is reflected on a water drop and is cast into a photodetector, places high demands on the light source used. The light irradiated for illuminating the window should be as homogeneous as possible so that the detection quality is not made a function of a location of the water drop. Lastly, the entire sensor system should be manufactured as cost-effectively as possible and should be easy to install. For this purpose, the light source should have a high degree of efficiency so that a major portion of energy supplied to the light source is converted into light, and the generated light should strike a predetermined sensitive surface of the window. A high intensity of the light striking the window is advantageous.